Wafer-to-wafer (W2W) bonding is deployed in a wide range of semiconductor process applications for forming semiconductor devices. Examples of semiconductor process applications where wafer-to-wafer bonding is applied include substrate engineering and fabrication of integrated circuits, packaging and encapsulation of micro-electromechanical-systems (MEMS) and stacking of many processed layers (3D-integration) of pure microelectronics. W2W bonding involves aligning two or more wafer surfaces, bringing them in contact and forming a strong bond interface between them. The overall processing yield and manufacturing cost of the so produced semiconductor devices and ultimately the cost of the electronic products that incorporate these devices depend greatly upon the quality of the wafer-to-wafer bond. The quality of the W2W bond depends upon the uniformity and integrity of the bond strength and the preservation of the alignment of the wafers across the wafer bond interfaces.
There are a number of wafer-to-wafer bonding methods, including direct/fusion/oxide wafer bonding, thermocompression bonding, adhesive bonding and metal diffusion bonding, among others. Direct wafer bonding refers to a process where two separate wafer surfaces are brought into contact and are bonded without any intermediate adhesives or external force. The initial bond strength is usually weak, and therefore a subsequent annealing step is generally carried out to strengthen the bond. The direct wafer bonding process can be viewed as a three-step process, including surface activation, room temperature bonding and annealing. The room temperature bonding, also known as pre-bonding is based on inter-atomic and intermolecular forces, also known as Van-der-Waals forces, hydrogen or water bridges. These forces are relatively weak. However, in many cases, a spontaneous bonding of two clean and flat surfaces occurs when initiated only in one single point. Typically the bonding is initiated in the center or at the edge. Once the bonding is initiated a so-called bonding front propagates across the bonding interface.
As was mentioned above, a significant parameter of the bond quality is the preservation of the initial alignment of the wafer surfaces. Several alignment wafer methods have been suggested that produce submicron alignment accuracy of the wafers. However, the follow-up steps of the bonding process distort the submicron accuracy of the initial alignment resulting in a final product where the wafer alignment may be degraded to more than several micron accuracy level. Accordingly, there is a need for an improved semiconductor wafer bonding operation that maintains the initial accurate alignment of the wafers throughout the entire bonding process.